Filters have been devised in the past to remove various particulate contaminants from a moving gas stream, and specifically for air streams. Such contaminants may, for example, include dust, lint, smoke, pollen or dander. For instance, in conventional Heating, Ventilating and Air Conditioning (HVAC) Systems, a filter is inserted into the air stream to remove contaminants carried along in the air stream. The filtered air may then be recirculated throughout the HVAC system.
Conventional air filters generally include a filter media constructed from a porous material, such as open cell foam or a non-woven web of materials such as fiberglass. As a stream of air is induced to flow through the filter, particulate contaminants are entrapped or separated from the air stream and accumulated on the filter media. After a period of use in this manner, the accumulated contaminants begin to interfere with the flow of the air stream through the filter. The increased pressure drop across the filter impedes the airflow through the air filter and potentially reduces the effectiveness of the entire HVAC system. This requires that the air filter be removed and either cleaned or replaced with a new air filter. However, typically the filter is hidden from view during normal use, making it difficult to determine the condition of the filter without removing it from the HVAC system for visual inspection. Visual inspection may be done periodically or on a predetermined schedule. However, visual inspection alone may not be entirely reliable since a filter may appear to be fully loaded with contaminants, when in fact only a portion of the capacity of the filter has been utilized.
However, devices have been provided in the past that generate a change signal to alert a user to the clogged condition of the filter and indicate the need for inspection, cleaning or replacement. Such devices include simple pressure sensors that measure the pressure on the downstream side of the filter and generate a signal if the pressure drops below a predetermined point. In another example, U.S. Pat. No. 4,321,070, issued to Bede, provides a whistle mounted in the filter. When the filter becomes sufficiently clogged, the air stream is induced to flow through the whistle portion at an increasing rate until an audible signal is produced. U.S. Pat. No. 2,782,747, issued to Alderfer, U.S. Pat. No. 2,753,831, issued to Davies, and PCT patent application Ser. No. 88/01050 to Horowitz, all also disclose an audible change signal device.
U.S. Pat. No. 2,804,839 discloses a device external to the filter that produces both an audible change signal and visual change signal responsive to an elevated differential pressure drop on either side of the filter. The visual signal is evident external of the filter and does not require removal of the filter itself and visual inspection. However, this device is complicated and expensive relative to the price of the air filter itself and thus does not lend itself to widespread use.
U.S. Pat. No. 3,027,865, issued to Kuntz et al. incorporates a diaphragm that shifts position responsive to a change in the pressure drop across an air filter. The shift in the position of the diaphragm may be used to provide a visual signal or to close an electrical contact in order to provide an electrical signal externally of the filter. U.S. Pat. No. 3,916,817 also discloses a device for providing a visual change signal.
However, none of these existing air filter change indicators are completely satisfactory. For instance, those that require elaborate and complicated apparatus to provide a change signal are relatively expensive, and may require setup at installation and periodic maintenance to ensure reliable operation. Those devices that depend on a reduction in airflow to provide an audible signal may be susceptible to clogging by the very contaminants that the air filter is designed to entrap. It is also generally not desirable to require modification to the air circulation system or to the air filter, as many of these approaches teach.
Moreover, all of these devices provide only one signal. That is, they only indicate when the air filter has reached a predetermined level of lowered performance. It may be desirable in certain situations to have a change indicator that provides a continuous and progressive indication of the performance of the filter.